It is generally desired to sterilize medical instruments, such as endoscopes having long, narrow lumens, and dental tools, before use. In medical facilities, where instruments need to be used several times per day on different patients, it is important not only to sterilize the instruments between patients to prevent cross-contamination, but to do so quickly and economically without damaging the instruments.
Several different methods have been developed for delivering sterilant vapor to a chamber for sterilizing medical instruments or other loads. In one option, the "deep vacuum" approach, a deep vacuum is used to pull liquid sterilant into a heated vaporizer; once vaporized, the sterilant is propelled by its vapor pressure into the evacuated chamber. In another option, the "flow-through" approach, vaporized sterilant is mixed with a carrier gas and delivered to the sterilization chamber under slightly negative or position pressure.
In yet a different approach, the sterilant vapor is introduced into the chamber under a combination of deep vacuum and flow-through conditions, in one cycle, to obtain a more efficacious sterilization than is achieved by the prior flow-through process or static, deep vacuum process alone. The combination vacuum/flow-through sterilization method is disclosed in commonly assigned, copending application U.S. Ser. No. 07/851,415, entitled "Sterilization Method for Multicomponent Sterilant," now abandoned in favor of copending continuation application Ser. No. 08/279,688, filed on Jul. 25, 1994 and incorporated by reference herein. In one embodiment described therein, a deep vacuum is first drawn in the sterilization chamber, followed by the injection of pulses of the sterilant vapor until a higher, second subatmospheric chamber pressure is reached. After sterilant vapor has been allowed to permeate the chamber and its contents, sterilant vapor is injected in pulses and flowed with a carrier gas into, through, and out of the chamber, during a transition phase, in which the chamber pressure reaches a higher third subatmospheric pressure, and then a fourth subatmospheric pressure, which is equal to a higher than the third subatmospheric pressure. The cycle then enters a flow-through phase at the fourth subatmospheric pressure, which includes successive alternating periods of sterilant and carrier gas flow and discontinuance of such flow. Sterilant vapor and carrier gas are preferably injected in successive pulses during the transition and flow-through phases, to increase sterilant penetration. At the end of the flow-through phase, there are one or more aeration steps to remove sterilant from the chamber.
When the above-described vacuum/flow-through method is used to sterilize a lumened instrument having at least two open ends and a fluid flow path between the open ends, one end of the instrument may be fluidly coupled to either the exhaust port or inlet port of the chamber. Sterilant vapor is thereby flowed through the lumens and also bathes the external surfaces of the instrument, when sterilant vapor is carried into and exhausted from the chamber.
If various types of lumened instruments, in particular, long and narrow endoscopes, are sequentially placed in and coupled to the sterilization chamber, and subjected to the same combination vacuum/flow-through sterilization cycle, the different orifice sizes, lengths, and shapes of the instruments may present varying degrees of restriction to the flow of carrier gas and sterilant vapor. The different instruments may thereby produce different final pressure levels and rates of chamber pressure increases as the sterilization cycle progresses from deep vacuum to flow-through conditions. The actual chamber pressure levels reached during the cycle phases will also depend on the day-to-day atmospheric pressure conditions.
It is desired to effectively and automatically control the amount of sterilant vapor delivered to a sterilization chamber, during the varying chamber pressure conditions which may be experienced during a sterilization cycle, or from cycle to cycle, to maximize sterilant vapor exposure. The level of sterilant vapor, however, should not exceed its saturation limit under the sterilization conditions. Otherwise, sterilant will condense, decreasing the amount of sterilant vapor available for sterilization. Also, condensed sterilant, such as hydrogen peroxide may degrade or harm the contents of the sterilization chamber. Synthetic materials, such as are employed in flexible endoscopes, for example, may be damaged by condensed hydrogen peroxide.
There is a need for a sterilization method which automatically provides optimum sterilant vapor exposure under varying pressure conditions which may occur within a vacuum combination flow-through cycle, for example, and/or from cycle-to-cycle, with different instrument loads and different atmospheric pressures. There is also a need for a sterilization method which avoids condensation resulting from exceeding the saturation limit of the sterilant vapor. There is a further need for an efficient sterilization method that can be economically implemented, with increased instrument through-put.